1. Field of the Invention
The present application relates to the field of electronics, and more particularly, to methods of forming flip chip bumps and related structures.
2. Description of the Related Art
A Solder On Pad (SOP) process is a technique of applying solder directly to an electrically conductive pad of a substrate to form a flip chip bump thereon. A SOP process encompasses both a solder mask defined (SMD) SOP process and a non solder mask defined (NSMD) SOP process.
In a SMD SOP process, openings are formed in a soldermask. Electrically conductive pads of the substrate are exposed through the soldermask openings. A solder screen process is used to apply solder paste within the soldermask openings. The solder paste is reflowed (heated to a melt and then cooled) to form flip chip bumps in the soldermask openings and electrically connected to the pads. However, if the size of the pads and thus solder mask openings becomes too small, i.e., the bump pitch becomes too small, the solder screen process cannot be reliably used to fill the soldermask openings. Unfortunately, this limits the use of a SMD SOP process for high density substrates.
In a NSMD SOP process, a solder screen process is used to selectively apply solder paste to electrically conductive pads of a substrate. The solder paste is reflowed to form flip chip bumps electrically connected to the pads. However, if the bump pitch becomes too small, the solder screen process cannot be reliably used due to shorting. Again, this limits the use of a NSMD SOP process for high density substrates.